Ryanair has announced a 21% year-on-year fall in first quarter profits. Photograph: Chris Radburn/PA

Ryanair has warned that hot weather in northern Europe has hit its summer bookings and prevented the carrier from pushing through a double-digit fare increase.

Announcing a 21% year-on-year fall in first quarter profits to €78m (£67m), the Irish carrier admitted that the heatwave in Britain and elsewhere had meant fewer high-revenue late bookings in recent weeks. It said the effect would be felt in the next quarter's results – traditionally the period where the airline generates the most revenue and profits.

Deputy chief executive Howard Millar said there had been a "weakness" in late bookings this summer, which were normally "the cream" for Ryanair, as he blamed the hot weather keeping people from wanting to book a holiday abroad. He said average fares would have been almost 10% higher without the heatwave, adding: "Our cream will be a little thinner. We'll just have to get by."

Despite the fall in bookings, Millar claimed the airline would make up the shortfall from the three months to 30 June, where profits were dampened by a late Easter and air traffic controller strikes in France.

Millar also pointed to the impact of high fuel prices, which make up 47% of the airline's costs. He said the company had made efficiencies to reduce the bill by 15% to €170m – including flying its planes slower. He claimed that the unspecified reduction in speed from about 600mph was saving fuel at a difference in journey time of just a couple of minutes per hour, but stressed there were no safety issues. Millar said: "When you tell everyone you are flying slightly slower the first thing people say is the plane will fall out of the sky."

The fuel bill was also lower due to a large reduction in passengers checking in luggage, after charges drove the proportion down to about 30% – a change he said represented 40m fewer items a year on Ryanair planes. Millar said Ryanair would continue to expand its geographical reach as its fleet expands from 290 to 410 planes over the next six years. He said they would be interested in starting flights to Turkey provided an open skies agreement was reached with the EU, and elsewhere in the periphery of Europe. Millar also confirmed that the airline had been in discussion with the Israeli government about new routes.

The airline expects a ruling from the UK competition commission on 13 August, which may see Ryanair ordered to sell down its stake in Aer Lingus. Millar said it was a "bizarre situation", and accused the commission of wasting "[British] taxpayer money investigating two Irish carriers". However, he said the airline would resist as long as possible in the hope it could eventually take over its Irish rival and pursue a "dual-brand strategy", likening its approach to manufacturers who sell "the same soap powder" for more money in different packaging.